Process for refining lead



"Patented Nov. 3, 1942 rmocE-ss Foa nnrnvmc. LEAD Yurii E. Lebedefl, Metuchen, N. J., assignor to American Smeltlng and ltefining Company, New York, N. Y., a corporation of New Jersey No Drawing.

This invention relates to the art of refining lead to remove bismuth.

Various processes havebeen offered over the yearsfor debismuthlzing lead with alkali, alkaline earth, rare earth metal reagents and, in

the ease of argentiferous bullion, it has been proposed to make the refining operation av continuous one by first desilverizing with zinc and then debismuthizing with alkaline earth metal, the excess or residual zinc and alkaline earth being thereafter removed to yield refined lead.

Following the discovery that calcium and mag nesium are decidedly more efilcacious in removing bismuth when used conjointly than when either one is used alone, the finding was promptly incorporated in plant practise by severallarge lead refineries and while'it was possible to employ the process immediately following the removal of the silver-zinc dross in the desilverizing step, the'procedure generally followed was to dezinc the desilverized lead prior to debismuthizing with calcium and magnesium in order v to retain the bismuth drosses free from zinc.

The calcium-magnesium process is ordinarily employed commercially for refining lead down to approximately 0.02% Bi although the process is fully capable of attaining much lower bismuth contents as shown in United States Letters Pat- Application September so, 1941, Serial No. 413,031

Claims.

desilverization; in the case of softened lead containing no silver or of desilverized lead which has been dezinced, the proper zinc content for the instant process can be easily arrived at simply by adding the requisite zinc to the lead. In any case the zinc content of the lead is adjusted to fall within the limits of from 0.015%

out No. 1,853,540 granted April 12, 1932 to Jesse O. Betterton. However, for the production of lead having bismuth finals materially below 0.02%, the use of antimony after the calciummagnesium treatment is much preferred in commercial practise. Details of this latter process are to be found in United States Letters Patent Nos. 2,056,164 and 2,101,975 granted Jesse O. Betterton and 'Yurii E. Lebedefl on October 6, 1936 and December 14, 1937, respectively.

The present invention provides a new and improved process for refining lead to 0.005% or less of bismuth using calcium and antimony and is primarily based onthe discovery= that zinc, when present in the lead within certain critical limits,

' exerts a remarkable influence upon the debismuthizing action of calcium and antimony thereby making it possible to attain low bismuth finals oi' the order above-indicated without resort to other or additional reagents,

In employing the new process, adjustment of the zinc content of the lead to be debismuthized to a valuewithin the critical limits of the invention is, of course, a requirement of first importance. In the case of softened argentiferous bullion the proper zinc content is readily at-- to 0.2% and preferably within the narrower range of 0.03% to 0.1% inasmuch as experience to" date indicates that'the lowest bismuth finals are obtained with a zinc content in the neighborhood of 0.05% or approximately one-tenth of the eutectic figure. l

Having adjusted the zinc content to within the proper limits, the metal is then treated with calcium and antimony for bismuth removal.

The latter operation is appropriately accomplished as hereinafter outlined although it will be understood that other specific modes of operation, known or occurring to those skilled in the art, may be employed.

The following specific example will serve to illustrate the invention. Refined lead containmg 0.21% bismuth was melted and drossed in a cast iron kettle and a zinc content of 0.05% imparted thereto by the addition of the requisite amount of zinc metal, the bath being held at 850 F. and the melted zinc thoroughly mixed with the lead.

Tothis molten lead analyzing 0.21% Biand 0.05% Zn, there was added an amount of calcium equivalent to 3.1 lbs. Ca/T, said calcium being added as a 4% Ca-Pb alloy and allowed to melt at a bath temperature of 780 F. The contents of the 'kettlewere then mixed for two minutes and the dross skimmed 700 F. The bath wasthen cooled to freezing, skimmed and one rim pulled from the kettle.

Antimony metal (sized to 5, on 20 mesh) was then added to the lead via the vortex created by the mixing machine, the addition'a'mounting to 0.33 lb. Sb/T on the basis of the original lead and being added at practically the freezing temperature of the bath as indicated by lead crystals on the bottom of the" kettle. Upon mixing for three minutes the temperature was raised to 675 F. and the dross skimmed. Two further additions of antimony metal of 0.33 lb. Sb/T were made in the identical manner and after skimming the last dross the bath was cooled to freezing,

skimmed and one rim pulled. Analysis showed the metal to have a bismuth content of only Aapreviously brought out, it is essential that tained by pa a y e i 's the e d i owinz as the zinc content of the lead fall within certain specified limits. This will be abundantly clear .rrom an inspection of the results of ten runs on lead containing 0.21% bismuth but with varying percentages of zinc as set forth in the table below.

In runs Nos. 1 to 6 inclusive the procedure followed was the same as that of the specific example just described. The procedure of runs Nos. '7 to 10 inclusive was simflar except that instead of making the antimony addition in three stages,

a single addition of 0.9 lb. Sb/T was made in each case.

iTable illustratingthe efiect of varying amounts of zinc upon bismuth removal While bismuth finals of approximately 0.005%

will ordinarily suffice, it is to be understood that the invention may be practised to attain much lower finals.

' Thus, in a particular instance, a zinc content of 0.05% was imparted to lead containing 0.21%

bismuth. This zinc-bearing lead was then de-' bismuthized with 3.1 lbs. Ca/T and three additions of antimony each amounting to 0.33 lbs.

Sb/T. The resulting metal was then further treated with calcium amounting to .5 lb. Ca/T of said metal and two antimony additions of 0.48

lb. Sb/T each. The bath was mixed, skimmed, etc., throughout the operation in the usual manner such as hereinbefore described. After skimming the last antimony dross, freezing and pulling the' rim, the refined lead in the kettle was reheated, sampled and found to contain only 0.0019% of bismuth.

From the foregoing disclosures, it will be readily apparent that the present invention provides improvements in debismuthizing lead with cal-' cium and antimony which permit the ready attainment of low bismuth finals with these reagents. Strange to say the same is not true when applied to. debismuthizing with calcium and magnesium followed by antimony. In that case, the presence of zinc has been found to adversely af-. feet the debismuthizing effect of antimony, the presence of only a, few thousandths per cent zinc serving to raise the bismuth final and the presence of only a few hundredths per .cent of zinc resulting in complete nullification of antimonys effectiveness as a debismuthizer.

Besides affording a practical process for attaining low bismuth finals, the invention offers other advantages. Thus, it has been found that the practice of the invention greatly decreases the decreases in antimony dross in the'neighborhood of 40-50 per cent have been repeatedly obtained.

While certain specific disclosures have been made to explain and illustrate the invention, it will be understood that various changes in operating procedure may be indulged in within the scope of the invention. In this connection, however, it is to be pointed out that while the proper zinc content may be imparted to the bath either before or after the addition of calcium, care .should be exercised to make it before the addition of antimony; otherwise one cannot expect to secure the results hereinbefore set forth.

What is claimed is:

1. The process for refining lead which comprises imparting an effective zinc content to the lead but not exceeding 0.2%, and debismuthizing the zinc-bearing lead with calcium and antimony.

2. The process for refining lead which com-.

prises imparting an efiective zinc content to the lead but not exceeding 0.2.%, debismuthizing the zinc-bearing lead with calcium and antimony,

and further debismuthizing the lead so refined with further additions of calcium and antimony.

3. The process for debismuthizing lead which comprises reacting a molten bath of the lead with calcium, thereafter imparting a zinc content of from 0.015% to 0.2% to the bath, and subsequently treating the zinc-bearing bath with antimony.

4. In a process for debismuthizing lead with calcium and antimony, that improvement which consists in imparting a zinc content of from 0.015% to 0.2% to the lead at least prior to the addition of antimony.

5. The process for refining lead which comprises debismuthizing the lead with calcium and antimony in the presence of an effective amount of zinc substantially less than that of the lead zinc eutectic.

6. In the art of refining lead, that improvement which consists in reacting calcium and antimony upon molten lead containing approximately 0.05% of zinc.

'7. The process for refining lead which comprises imparting thereto a zinc content of from 0.015% to 0.2%, and thereafter debismuthizing the resulting product with calcium and antimony as reagents.

8. The process for refining lead which comprises imparting a zinc content of from 0.03% to 0.1% to the lead, and debismuthizing the zincbearing lead with calcium and antimony.

9. The process for refining lead which comprises imparting thereto a zinc content of from 0.03% to 0.1 and debismuthizing the resulting zinc-bearing lead with calcium and antimony, the antimony being added in a plurality f portions and after the calcium addition. 1

10. The process for refining softened, argentif erous lead which comprises desilverizing the lead with zinc, dezincing the desilverized lead only partially but at least to 0.2%, and debismuthizing the partially dezinced lead with calcium and antimony. YURII E. LEBEDEFF. 

